Field of the Invention
The invention of the instant application relates to a device for automatically catching a torn material web running through a rotary printing machine, the device having a pair of clamping rollers, respectively, beneath and above the material web, the rollers being rotatingly entrained and being able to clamp the web when a web tear is detected.
The invention has a particularly advantageous application in the printing sector, in that it can avoid jamming by winding a material web on a blanket cylinder of the last printing unit, when a material web running continuously through a rotary offset printing machine is torn.
The offset printing units usually have four parallel cylinders which are in outer surface contact with one another via one of the jacket lines thereof. The printed material web runs between the two central cylinders, the so-called "blanket cylinders", which are covered by a blanket formed by an elastomer-coated canvas. Because the printing ink for the printing of the material web is usually highly viscous, the material web consequently adheres to the damp blanket cylinders. When the printing machine is in operation, the material web is tensioned or tautened to a very pronounced extent by rollers such as infeed rollers located upline of the printing units, and cooling rollers arranged downline of the printing units and a dryer.
The tensioned material web is then separated or stripped off the damp blanket cylinders. If the web becomes torn, the separation thereof from the cylinders abruptly stops. Furthermore, when hot-drying offset printing inks are used, there is provided in the printing machine a dryer having a length of from 8 to 12 meters that serves for vaporizing the solvent of the ink applied to the material web and for polymerizing the resin thereof.
The dryer is arranged downline of the printing units and is traversed by the continuous printed material web that does not have to be supported by rollers.
Because the dry material web is less elastic than the damp web, the dry material web may become torn in the dryer.
If the material web tears inside the dryer, it usually winds itself around a blanket cylinder of the last printing unit due to the adhesive capacity of the ink applied to the web, the adhesive capacity, in specific cases, being capable of producing very large adhesive forces. Following a given number of rotations, if the number of material layers has reached a sufficient value, the torn web that has been wound around the blanket cylinder may damage the blanket or the cylinder.
Before the printing machine runs down completely, there is a very great risk of two material layers winding around the blanket cylinder during each revolution of the latter, one material layer coming from the printing unit and the other coming from the dryer.
In order to avoid the foregoing disadvantages, the European Patent 0 092 659 has disclosed heretofore a so-called turn-back safety device of the type described in the introduction hereto for a torn material web.
This heretofore known device is located between the last printing unit and the dryer. A main part of this device is formed by a pair of clamping rollers including a fixed roller located laterally beneath the continuous material web and a pressure-exerting roller arranged above the web, the rollers, respectively, being entrained with the material web at the running speed of the latter.
In normal operation, the rollers are kept apart from one another during the running of the paper, so that the printed damp web can run past the rollers without contact, and no smearing occurs. When the paper web tears, the tearing is reliably detected by optical sensors which are arranged at the outlet of the last printing unit, and a releasing signal is directly transmitted to a holder for holding back the spaced-away pressure-exerting roller.
The pressure-exerting roller is then released and brought into the clamping position for clamping the torn material web.
As long as the printing machine is not switched off, the material web oncoming from the last printing unit continues to run and winds around one of the two clamping rollers of the respective device, instead of winding around one of the blanket cylinders of the printing unit.
The diameter of the roller around which the paper winds becomes greater, while the other roller yields so as to permit the material take-up to occur.
The disadvantage of such a device resides mainly in the fact that the rotating and laterally displaceable arrangement of one of the clamping rollers for positioning it in a spaced-away position or in a position wherein it engages the material web is quite complex, and the rotary printing machine thus becomes more sluggish.
Furthermore, this complex construction means that the operator does not have ready access to the respective clamping roller.
Furthermore, a device of this type has a relatively lengthy response time resulting from the physical displacement of one of the clamping rollers relative to the other.